Public awareness of the discharge of industrial wastes has resulted in governmental and private development of efficient, economical and safe procedures for the destruction of toxic organic waste. Alternatives to the traditional use of thermal incineration include supercritical water oxidation, photochemical degradation, sonochemical oxidation and electrochemical incineration.
Supercritical water oxidation is performed above the critical point of water (374.degree. C., 218 atm) in the presence of O.sub.2 or H.sub.2 O.sub.2. Organic species only slightly soluble in water are miscible with supercritical water. The literature contains descriptions of reaction mechanisms, kinetics and engineering aspects of supercritical water oxidation applied to numerous organic pollutants including: phenol, 1,3-dichlorobenzene and benzene, pyridine, acetic acid, 1,4-dichlorobenzene, chlorophenols, pulp and paper mill sludge, and explosives. Major reaction products are water, carbon dioxide and inorganic salts. Supercritical water oxidation is well suited for destruction of large volumes of toxic organic waste; however, for disposal of small quantities of toxic organic waste, supercritical water oxidation is not considered feasible economically. Therefore, evaluation of less costly methods is appropriate.
Recently, interest in photochemical degradation of toxic organic waste in aqueous media has expanded rapidly. The primary oxidant is the photogenerated hydroxyl radical formed on semiconductor metal oxide surfaces. Typically, TiO.sub.2 powder is the semiconductor used because it is inexpensive, insoluble under conditions used in photochemical degradation, stable and non-toxic. The literature of photochemical degradation describes applications to chlorophenols, dichloroacetate and oxalate, 4-chlorophenol, humic acids, dichlorophenols, benzene, phenol, dimethoxybenzene and toluene. Applications of photochemical degradation appear most suitable for solutions having low turbidity.
Sonochemical oxidation has been used for degradation of phenol and humic acids; and of 4-chlorophenol, 3,4-dichloroaniline and 2,4,6trinitrotoluene. The primary reaction in sonochemical oxidation is the pyrolysis of solute present in bubbles generated by acoustical cavitation. Secondary reactions also occur as a result of interactions of solute with hydroxyl radicals and hydrogen atoms produced by the sonication of water.
Electrochemical incineration is an alternative to the degradation methods above described. This is a waste remediation process whereby oxygen atoms are transferred from H.sub.2 O in the solvent phase to the oxidation product(s) by direct or indirect reactions on the anode surface. This procedure is attractive for low-volume applications such as confined living spaces, e.g., spacecraft, and research laboratories. The prior art has described successful electrochemical incineration of waste biomass using Pt and PbO.sub.2 electrodes. The major advantages of electrochemical incineration over thermal incineration include: absence of CO and NO.sub.x generation, and low operating temperatures. Because of the high cost of Pt and the toxicity of lead salts, two Swiss groups compared PbO.sub.2 and Pt electrodes to SnO.sub.2 -film electrodes doped with Sb(V) ("Sb-SnO.sub.2 "). Both Swiss groups demonstrated that phenol is removed from aqueous solution more efficiently with Sb-SnO.sub.2 anodes than with Pt and PbO.sub.2 anodes. Their work also indicated that for Pt anodes, oxidation stops with the formation of small carboxylic acids, e.g., maleic, fumaric and oxalic. More recently, Pt, IrO.sub.2 /Ti, and Sb-SnO.sub.2 /Ti anodes were compared and a mechanism for the electrolysis of organic compounds was proposed. These and other descriptions of electrochemical incineration have been reviewed in the literature; advantages of electrochemical incineration include: versatility, energy efficiency, amenability to automation, environmental compatibility and low cost.
The major challenge for future development of electrochemical incineration is the discovery of nontoxic anode materials and electrolysis conditions that can achieve conversion of toxic organic waste to innocuous products with high current efficiencies. Other desirable electrode properties include low cost, lack of toxicity, high stability and high activity. The matter of current efficiency is especially pertinent because the desired O-transfer reactions require the anodic discharge of H.sub.2 O to produce adsorbed hydroxyl radicals (OH.sub.ads). However, a high surface excess of the OH.sub.ads species leads to evolution of O.sub.2, an undesired product. Previous work has demonstrated that electrodes comprised of Fe(III)-doped .beta.-PbO.sub.2 films on Ti substrates ("Fe-PbO.sub.2 /Ti") are quite stable in acetate buffered media (pH 5) and offer significantly improved catalytic activity over pure .beta.-PbO.sub.2 film electrodes for conversion of CN.sup.- to CNO.sup.- under potentiostatic conditions as well as the anodic degradation of p-benzoquinone under galvanostatic conditions.